DF Bit Override Functionality with IPsec Tunnels

Last Updated: October 28, 2011

The DF Bit Override Functionality with IPsec Tunnels feature allows you to configure the setting of the DF bit when encapsulating tunnel mode IPsec traffic on a global or per-interface level. Thus, if the DF bit is set to clear, routers can fragment packets regardless of the original DF bit setting.

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Prerequisites for DF Bit Override Functionality with IPsec Tunnels

IPsec must be enabled on your router.

Restrictions for DF Bit Override Functionality with IPsec Tunnels

Performance Impact

Because each packet is reassembled at the process level, a significant performance impact occurs at a high data rate. Two major caveats are as follows:

The reassemble queue can fill up and force fragments to be dropped.

The traffic is slower because of the process switching.

DF Bit Setting Requirement

If several interfaces share the same crypto map using the local address feature, these interfaces must share the same DF bit setting.

Feature Availability

This feature is available only for IPsec tunnel mode. (IPsec transport mode is not affected because it does not provide an encapsulating IP header.)

Information About DF Bit Override Functionality with IPsec Tunnels

Feature Overview

The DF Bit Override Functionality with IPsec Tunnels feature allows you to specify whether your router can clear, set, or copy the Don't Fragment (DF) bit from the encapsulated header. A DF bit is a bit within the IP header that determines whether a router is allowed to fragment a packet.

Some user configurations have hosts that perform the following functions:

Set the DF bit in packets they send

Use firewalls that block Internet Control Message Protocol (ICMP) errors from outside the firewall, preventing hosts from learning about the maximum transmission unit (MTU) size outside the firewall

Use IP Security (IPsec) to encapsulate packets, reducing the available MTU size

If your configurations have hosts that prevent you from learning about the available MTU size, you can configure your router to clear the DF bit and fragment the packet.

Note

In compliance with RFC 2401, this feature can be configured globally or per interface. If both levels are configured, the interface configuration will override the global configuration.

DF Bit Setting Configuration Example

In following example, the router is configured to globally clear the setting for the DF bit and copy the DF bit on the interface named FastEthernet. Thus, all interfaces except FastEthernet will allow the router to send packets larger than the available MTU size; FastEthernet will allow the router to fragment the packet.

RFCs

No new or modified RFCs are supported by this feature, and support for existing standards has not been modified by this feature.

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The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

This feature allows users to specify whether their router can clear, set, or copy the Don't Fragment (DF) bit from the encapsulated header. A DF bit is a bit within the IP header that determines whether a router is allowed to fragment a packet.

The following commands were introduced or modified: cryptoipsecdf-bit.

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